1. Technical Field
This invention relates to a high strength steel weld having a tensile strength (TS) of at least 750 MPa along with improved low temperature toughness and improved resistance to cold cracking, and a welding method capable of forming such a weld. A steel weld and a welding method according to the present invention are suitable for use in high strength steel structures such as welded steel pipes for pipelines and other uses, offshore structures, pressure vessels, tanks, and ship hulls.
2. Related Art
In a pipeline for transporting natural gas, crude oil, or similar material over long distances, if the operating pressure is increased, the transport efficiency of the pipeline is improved and transport costs are decreased. In order to increase the operating pressure, it is necessary to either increase the wall thickness of the pipe forming the pipeline or increase the strength of the material of which the pipe is formed. However, if the wall thickness of the pipe is increased, the efficiency of field (on-site) welding decreases, and it may become necessary to strengthen the foundation supporting the pipeline due to the increased weight of the pipe.
For this reason, there is an increasing need for higher strength welded steel pipe. In recent years, X80 grade welded steel pipe having a yield strength (YS) of at least 551 MPa and a tensile strength (TS) of at least 620 MPa has been standardized by the American Petroleum Institute (API) and has been put to actual use.
Furthermore, in recent years, high strength welded steel pipe superior to X80 grade is being developed based on the manufacturing technology for X80 grade welded steel pipe using a high strength steel having a tensile strength of 950 MPa or higher along with improved low temperature toughness and field weldability.
With such high strength welded steel pipe, it is important to not only guarantee the desired properties of the steel pipe base metal and the weld, but to also prevent cold cracking in the weld metal of the weld. In general, the strength of weld metal in a steel weld must be higher than the strength of the base metal. Accordingly, as the strength of a steel pipe base metal increases, it is necessary for the strength of a weld metal to also increase. Normally, the strength of a steel pipe base metal can be easily increased by adjusting the working conditions for hot rolling (by using controlled rolling and accelerated cooling). The strength of a weld metal is determined by its chemical composition and the cooling speed following welding. Thus, when the welding conditions are fixed (such as when the thickness of a steel plate being welded is fixed), the strength of the weld metal is thought to be nearly entirely determined by the amount of alloying elements.
Therefore, in order to increase the strength of a weld metal, it is necessary to include a large amount of alloying elements which are effective for increasing the strength of steel. However, if a weld metal contains a large amount of alloying elements, an increased amount of hydrogen is taken up into the weld metal during welding, thereby significantly increasing the susceptibility of the weld metal to cold cracking.
In order to improve the resistance to cold cracking of a weld metal, U.S. Pat. No. 6,188,037 proposes to reduce susceptibility to hydrogen cracking by increasing the amount of oxygen in a weld metal. However, due to the increased amount of oxygen in the weld metal, not only does the toughness of the weld metal decrease, but it also becomes easy for defects such as slag inclusions and pinholes to occur in the weld metal, leading to a decrease in weld quality.
Japanese Patent No. 2,555,400 proposes a consumable nozzle electroslag welding method in which a weld metal having a prescribed chemical composition is formed and it is cooled by accelerated cooling at a cooling rate of 0.3-5° C./sec to a temperature of 500° C. or below. In the examples of that patent, the temperature at the completion of cooling is in the range of 500-300° C. In that method, the welding techniques which can be employed are restricted, and hence it cannot be applied to steel pipe, for example. In addition, the subject of that patent is a low strength steel, and it does not disclose a method of preventing cold cracking in a high strength steel, which is the subject of the present invention.